Summary The innate immune response is a double-edged sword; it is absolutely required for host defense, but unregulated, causes inflammatory disease. Diverse and potent mechanisms have evolved to recognize and counter invading microorganisms. These include a variety of pattern recognition receptors, including Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), Nod-like receptors (NLRs) and C-type lectin like receptors (CLRs) that recognize conserved molecular motifs on pathogens. While significant progress has been made in identifying the ligands detected by these receptors and the signaling cascades that they activate, a number of critical questions regarding the mechanisms that appropriately tailor the outputs of these pathways remain unanswered. Furthermore, the immune response to live pathogens is shaped by the interaction of multiple receptors and their cognate signaling pathways. The aggregate response is complex and cannot be predicted from analysis of each pathway in isolation; however, it is tractable using the tools of systems biology and forward genetics. Over the past 15 years members of this U19 consortium have collaborated using cross-disciplinary approaches to define the molecular mechanisms underlying the regulation of immune receptors and pathways. Their genetic approaches have also linked the pathways to pathogenesis and to immunity in vivo. These studies have also generated a significant body of work demonstrating cross-regulation between innate immune receptors. This U19 consists of two interrelated Projects that probe the innate immune response to infection. In Project 1, the Beutler laboratory will work in close collaboration with the Aderem, Nolan, and Ulevitch laboratories, taking a highly automated forward genetic approach to the analysis of innate immune signaling. In Project 2, the Aderem laboratory will determine mechanisms by which the TLR and type I interferon pathways cross-regulate each other. The Projects will be supported by three scientific Cores: The Signaling Core, will bring to bear several novel technologies for highly multiplexed molecular phenotyping of immune cells. The Data Management and Bioinformatics Core will support the individual Projects as well as the overall goals of the program through integrated computational analysis of all large-scale datasets. The Human Correlation Core will examine the relevance of mouse genes, demonstrated in Projects 1 and 2 to mediate innate immune functions, in the analogous human pathways.